1MRK 504 158-UEN A
The only requirement is that the power transformer winding is connected to earth in the star
point (in case of star-connected windings) or through a separate earthing transformer (in case
of delta-connected windings).
The low impedance restricted earth-fault protection REFPDIF is a winding protection function.
It protects the power transformer winding against faults involving earth. Observe that single
phase-to-earth faults are the most common fault types in transformers. A sensitive earth-fault
protection is therefore desirable.
A restricted earth-fault protection is the fastest and the most sensitive protection, a power
transformer winding can have and will detect faults such as:
•
earth faults in the transformer winding when the network is earthed through an
impedance
•
earth faults in the transformer winding in solidly earthed network when the point of the
fault is close to the winding star point.
The restricted earth-fault protection is not affected, as a differential protection, with the
following power transformer related phenomena:
•
magnetizing inrush currents
•
overexcitation magnetizing currents
•
load tap changer
•
external and internal phase faults which do not involve earth
•
symmetrical overload conditions
Due to its features, REFPDIF is often used as a main protection of the transformer winding for
all faults involving earth.
6.2.2.1
Transformer winding, solidly earthed
The most common application is on a solidly earthed transformer winding. The connection is
shown in figure 31.
Application manual
Section 6
Differential protection
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